It is widely known that MTBE can be prepared from isobutylene and methanol using an acidic ion exchange resin as a catalyst. An isobutylene source to be used therefor usually includes a hydrocarbon fraction having 4 carbon atoms (C.sub.4 fraction) which is a by-product produced from an ethylene plant, a so-called spent B--B fraction that is a C.sub.4 fraction from which 1,3-butadiene has been removed, a so-called FCC C.sub.4 fraction that is a by-product produced from a catalytic cracking facility which is widely employed in recent years, and the like. These C.sub.4 fractions will hereinafter be referred to "C.sub.4 hydrocarbon mixture".
In the production of MTBE from the C.sub.4 hydrocarbon mixture containing isobutylene and methanol in the presence of an acidic ion exchange resin, it is well known that a methanol feed exerts a great influence on an isobutylene recovery. In case when the methanol feed is less than a chemical equivalent for the reaction with isobutylene, by-production of dimers and trimers of isobutylene increases. Therefore, methanol is generally fed in excess, usually in an amount of from the chemical equivalent to twice the chemical equivalent to isobutylene to suppress formation of oligomers. However, feeding of excessive methanol results in an increase in cost for recovery of the excess. Even if the amount of methanol to be fed is less than the chemical equivalent amount, methanol unavoidably remains unreacted because the reaction between isobutylene and methanol is an equilibrium reaction. Thus, from the economical viewpoint, recovery of methanol is essential and has generally been conducted by washing extraction with water, distillation, and the like.
For example, U.S. Pat. No. 4,219,678 discloses a process for producing MTBE which comprises separating unreacted hydrocarbons from a reaction mixture, distilling the reaction mixture under pressure, and recovering pure MTBE from a bottom of a distillation column while recycling a methanol-containing distillate which is formed during the distillation to a reaction zone of methanol and isobutene. This process, however, has many problems. That is, since methanol is recovered taking advantage of azeotropy of methanol and MTBE, evaporation of the entrained MTBE requires a great deal of energy. Further, the distillation column should be operated under a high pressure in order to ensure higher recovery rate, which causes a significant increase of construction cost of facilities. Moreover, a heat source for high temperatures is required. In addition, in the separation of the unreacted hydrocarbons from MTBE, methanol is also separated as an azeotropic mixture with the unreacted hydrocarbons. Therefore, it is necessary to recover methanol from the distilled unreacted hydrocarbons by washing extraction with water and the like.
On the other hand, U.S. Pat. No. 4,544,776 discloses a process in which a reaction mixture immediately after withdrawal from a reaction zone is washed with water, and an extract containing methanol is distilled to separate methanol from water, said methanol being recycled to the reaction zone, while a raffinate left after the methanol removal is distilled to separate MTBE from unreacted hydrocarbons. This process, also, requires high energy since methanol is recovered by washing extraction with water, followed by distillation.